1. Field of the Invention
The present invention relates to an optical information recording medium, wherein recording or erasing information is conducted by changing configuration of atoms constituting a recording layer by irradiating light.
2. Description of the Related Art
A so-called phase-change type optical information recording medium has been well known as one of optical memory mediums, which enable to record, reproduce or erase information by irradiating a laser beam. Such a phase-change type optical information recording medium-uses transition between crystal and amorphous or between two crystalline phases of crystal 1 and crystal 2.
A thin film of chalcogen system alloy is commonly used as a material for recording layer of a phase-change type optical information recording medium. A thin film of GeSbTe system alloy or AgInSbTe system alloy among the chalcogen system alloys has been practically applied for an optical information recording medium such as a rewritable optical disc.
A recording principle of such a rewritable optical disc is as follows. A recording layer immediately after being filmed is in an amorphous state and low in reflectivity. Therefore, the recording layer of the optical disc is heated by irradiating a laser beam first, and then whole area of the recording layer is conducted to be a crystalline state that is high in reflectivity. In other words, the optical disc is initialized. Generally, this initialization is conducted by irradiating a laser beam converged with a diameter of some ten to 100 μm on the optical disc that is rotating.
A part of the recording layer is melted by irradiating a laser beam on the initialized optical disc locally, and then cooled down rapidly. A phase of the part of the recording layer results in changing into an amorphous state. Accordingly, optical characteristics such as reflectivity, transmissivity, and birefringence index of the recording layer change in response to the phase change, and information is recorded.
Reproduction is conducted by detecting reflectivity difference between crystal and amorphous or phase difference between them by irradiating a weaker laser beam than that for recording.
Further, rewriting is conducted by overwriting on a prerecorded record mark without processing through an erasing process by injecting a recording peak power that is superimposed on an erasing power in low energy causing crystallization into the recording layer.
In the case of the GeSbTe system alloy except for the materials practically applied, it is commonly known that an eutectic composition of Sb and Te transforms its crystalline state to amorphous state or vice versa.
The Japanese Patent Application Laid-open Publication Nos. 1-115685/1989, 1-251342/1989, and 1-303643/1989 disclose the range of composition such as adding a third element, particularly, adding Ge into Sb70Te30, wherein suffixes 70 and 30 represent an atomic ratio.
However, the prior arts disclosed in the above-mentioned Japanese Patent Application Publications could not realize such an optical information recording medium that is adequate to recording and reproducing characteristics and a contrast ratio, and high in durability against reproduction light.
With respect to a recording method of an optical disc that is made from a phase change material, recording has been conducted by using a red laser beam having a wavelength of around 650 nm that has been used for recording DVD-ROM discs, or by using a laser beam having a wavelength longer than 650 nm. Recently, however, a semiconductor laser element emitting light having a wavelength of around 400 nm has been introduced into the market.
Further, a numerical aperture (hereinafter referred to as NA) of an objective lens has been increased, and resulted in enabling to converge a light beam furthermore.
Combining a laser beam having a shorter wavelength and an objective lens having a higher NA to use makes a spot diameter of a laser beam smaller and results in enabling to increase recording density of an optical disc more. Consequently, an optical disc system using a blue laser beam has been studied.
By using such an optical disc system using a blue laser beam, researching in and developing an optical disc, which is excellent in performances superior to those of the conventional optical disc system using a red laser beam, and realizes high density recording, is commonly practiced. It is demanded for such an optical disc to be capable of a blue laser beam having a shorter wavelength and to be able to record sufficiently even by a short pulse width, and further to be able to rewrite.
With respect to GeSbTe system materials in the neighborhood of eutectic composition out of conventional materials, it has been experimentally confirmed that the GeSbTe system materials enabled to be recorded and reproduced to some extent by using an optical disc system equipped with a blue laser beam.
Further, in order to improve recording and reproducing characteristics, it is also experimentally confirmed that a linear velocity corresponding to recording can be changed by controlling a crystallizing speed by adjusting balance of each amount of Sb and Te in compositional elements of the GeSbTe system materials.
Furthermore, an amount of Ge severely affects stability of recording materials. If the amount of Ge is within a moderate range, reproduction durability against a blue laser beam having a small beam diameter and a high energy density can be increased.
However, in order to perform recording in higher density, it is essential to improve recording and reproducing characteristics furthermore. In other words, increasing reproduction power and reducing jitter is essential for recording in higher density. On the contrary, in a case of an optical disc made from GeSbTe system materials as a recording material, there exists a characteristic limit with respect to increasing reproduction power and reducing jitter.